Refrigeration system with means to maintain a minimum condensing pressure



Nov. 20, 1962 K. M. GERTEIS REFRIGERATION SYSTEM WITH MEANS T0 MAINTAIN A MINIMUM CONDENSING PRESSURE Filed March 7, 1960 INVENTOR. KARL M GERTEIS By {M illnited rates 3,654,445 Patented Nov. 20, 1%52 REFREGERATKGN SYTEM WITH MEANS TO MAlNTAiN A MlNiMUlt i CUNDENSENG PRES:- SURE Karl M, Gerteis, Syracuse, NFL, assiguor to Carrier (Jorporation, Syracuse, N.Y., a corporation of Delaware Filed Mar. 7, 1969. Ser. No. 13,351 4 Claims. (ill. 62-174) This invention relates to a refrigeration system and more particularly to an improved refrigeration system of the type employing a thermostatic expansion valve wherein the refrigeration system is required to operate in its normal manner over a wire range of ambient conditions.

Refrigeration systems for use in applications wherein the anticipated load and ambient condenser temperature may vary over a wide range frequently employ thermostatic expansion valves to provide the degree of control of refrigerant flow which is required in such a system. A difiiculty arises however, in systems of this type in that substantial changes in ambient condittions require the expansion valve to operate over a relatively wide range of operating pressures and loads and a valve selected in accordance with this requirement is sensitive to small changes in superheat which is usually sensed by a bulb placed at the outlet endof the evaporator coil. Consequently, a subsantial change in refrigerant flow may be caused by a relatively small change in superheat sensed by the bulb. Therefore, the expansion valve over corrects for variations from the design conditions. This over correction may cause flooding of the evaporator coil in extreme cases and frequently creates acondition known 'as hunting wherein the expansion valve oscillates from a condition such that too much refrigerant is passed to the evaporator to a condition where too little refrigerant is passed thereto. As is Well known hunting is highly undesirable in a refrigeration system because the degree of control over the desired design conditions in the area being refrigerated or air conditioned is substantially lessened. In addtion, a thermostatic expansion valve having a wire range of orifice opening to accommodate substantial changes in the operating conditions of the system are relatively expensive.

it is therefore common practice to employ relatively small range thermostatic expansion valves in a refrigeration system. Small range thermostatic expansion valves provide relatively good regulation of refrigerant flow over a reasonably wide range of operating conditions while at t.e same time, they reduce the effect of hunting which is characteristic of expansion valves having a wider range. Likewise, the cost of the valve and consequently the refrigeration system is reduced by the use of a small range valve.

However, the use of a relatively small range'thermostatic expansion valve in a refrigeration system introduces a problem Which becomes serious under widely varying ambient conditions especially where an air-cooled condenser coil is directly exposed to ambient air temperatures which are substantially below those for which the system was designed. For example, when the outdoor temeprature to which the condenser coil is exposed drops to a relatively low temperature, the condensing temperautre and hence, pressure in a refrigeration system likewise drops. Since the condensing pressure has dropped, less refrigerant is passed through the thermostatic expansion valve for a given orifice opening. At the same time, however, the compressor is working against a lower head pressure and becomes relatively more efficient thereby pulling a lower evaporator pressure. If the cooling load is relatively low, as it often is under conditions of low outdoor ambient temperatures, less heat will be added it properly filled.

to the refrigerant flowing through the evaporator coil and the thermostatic expansion valve Will tend to maintain an orifice opening that will provide the proper degree of superheat for the refrigerant leaving the evaporator. The net result of these conditions is that the evaporator coil tends to operate at a a very low temperature and moisture from the surrounding air condenses and forms ice about the coil.

if, in addition, an expansion valve has been selected with a limited range, with the drop in head pressure below the level obtained with normal ranges of outdoor ambients, the valve even when wide open may not be able to supply suilicient liquid to the evaporator to keep This will result in further reduction of the evaporator temperature and freezing of the portion of the coil supplied with liquid, since the compressors pumping capacity can be brought into equilibrium with the rate of refrigerant evaporation only through a decrease of gas density and volumetric eihciency through an increase of the compression ratio by reduction of the evaporator pressure. The ice further insulates the coil from thesurrounding atmosphere and accentuates the condition which caused icin of the evaporator coil as well as lessening the capacity of the system.

Accordingly, it is an object of this invention to provide an improved refrigeration system of the type employa thermostatic expansion valve which will operate effectively over a wide range of ambient conditions.

it is a further object of this invention to provide a refrigerant reservoir in a refrigeration system of the type employing a thermostatic expansion valve in a manner such that a minimum condensing pressure within the range of the design conditions of the expansion valve is maintained under conditions of low ambient condenser air or liquid temperatures. 7

These ad other objects of this invention are achieved in the illustrated embodiment by connecting a compressor, a condenser, a thermostatic expansion valve and an evaplowing specification and attached drawing wherein:

The drawing is a schematic illustration of a refrigeration system embodying the principles of this invention.

Referring specifically to the drawing, a compressor 10, a condenser coil 11, a thermostatic expansion valve 12 and an evaporator coil 13 are connected in series to form a refrigeration circuit. A liquid line 25 connects condenser 11 to expansion valve 12. Condenser 11 may be of the air-cooled type, evaporative-cooled type or a We.-

'ter-cooled condenser of the tube in shell or coil in shell type.

Thermostatic expansion valve 12 is of a Well known type having a diaphragm l and a bulb 1'5. Pressure of the refrigerant downstream of expansion valve 12 or in other Words, adjacent the inlet of evaporator 13 is transmitted to the underside of the diaphragm. Pressure of a similar refrigerant in bulb 15 which is secured to the out et of evaporator 13 is transmitted to the upper side of the diaphragm. Consequently, if the temperature of the refrigerant leaving the evaporator is significantly higher than the saturation pressure of the refrigerant, diaphram 14 will sense the superheated condition of the refrigerant and move in a direction to open valve 12 and admit more refrigerant to the evaporator coil. Conversely, if too much refrigerant is being supplied to evaporator coil 13, the pressure transmitted from bulb will be substantially that of the saturation pressure of the refrigerant and the diaphram 14 will move in an upward direction due to the biasing action of the valve spring tending to reduce the flow of refrigerant to the evaporator. If the pressure drop through evaporator coil 13 is substantial, it may be desirable to employ a valve of the type wherein an equalizer line communicates with underside of diaphragm 14 to the outlet of evaporator coil 13 rather than with its inlet as shown in the drawing.

Refrigerant reservoir 16, comprising a closed tank, is provided with a line 21 which communicates the interior of reservoir 16 to the liquid line 25 of refrigeration circuit. Line 21 may be physically connected to liquid line 25 of the of the refrigeration circuit at any convenient point between expansion valve 12 and condenser 11.

As shown in the drawing, an electric heating element 17 may project into the interior of refrigerant reservoir 16 and is connected to a convenient source of electric current by means of a transformer 18 and the contacts of a thermostat 19. Alternatively, the heater may heat the side of the reservoir tank or the space around it. Thermostat 19 has a Sensing element 20 which may be secured to the outside of reservoir 16 in heat exchange relation with refrigerant in reservoir 16 by conduction through the tank wall. Sensing element 20 serves to close the contacts of the thermostat if the temperature of the refrigerant in the reservoir drops below a predetermined temperature. The particular temperature which is selected will be determined by the desired minimum condensing pressure or temperature which is to be maintained. If desired, reservoir 16 may be insulated from changes in ambient temperature by enclosing it within an insulated container 24. A purge line 22 having a manually operable valve 23 therein, may lead from the upper portion of reservoir 16 for the purpose of venting accumulated noncondensible gases to the atmosphere.

As an alternative arrangement, refrigerant reservoir 16 may be located either in the area to be conditioned or in any convenient area having a substantially constant predetermined temperature. In that event, insulated container 24 would be omitted because it would be desired to maintain the walls of reservoir 16 in heat exchange with the surrounding air. If a convenient area in which 'to locate reservoir 16 is available which has a sufliciently high and uniform temperature, it will be understood that heater 17 together with its associated circuitry may also be omitted. It will be appreciated that the alternative described requires the availability of an area having a temperature sufliciently high to maintain the desired minimum condensing temperature or pressure. Consequently, with the use of certain refrigeration systems, the use of an electric heater is mandated if a convenient area having a sufliciently high temperature is not available.

In operation, under normal conditions, refrigerant reservoir 16 is partially or preferably completely filled with refrigerant and heater 17 supplies heat to the refrigerant in the reservoir at an average rate which provides the desired reservoir refrigerant temperature. However, under normal conditions the condensing temperature and pressure are such that they exceed the vapor pressure of refrigerant that would be produced by the temperature maintained in the reservoir. It should be noted, however, that the pressure in the reservoir is the same as that of the liquid line. Likewise, expansion valve 12 and evaporator coil 13 are operating normally to maintain the area to be conditioned at the desired temperature and humidity and refrigerant flow is within a range of operation of the expansion valve.

Under abnormal operating conditions wherein the temperature of the ambient air surrounding condenser 11 is relatively low, it would be expected that the condensing pressure and evaporator pressure of the system would drop. However, if the condensing pressure begins to drop, it reaches a point where it has fallen below the vapor pressure of refrigerant produced by the temperature of reservoir 16 which maintains the refrigerant therein at a relatively constant temperature without regard to the ambient conditions. Consequently, refrigerant begins to vaporize in reservoir 16 and the vaporized refrigerant collects at the top thereof. The pressure of the vaporized refrigerant forces liquid refrigerant from reservoir 16 through line 21 into liquid line 25 of the refrigeration system thereby attempting to maintain equalization of the pressure in the reservoir and the condensing pressure.

If the thermostatic expansion valve is functioning properly, the evaporator can accept no additional refrigerant charge. Therefore, the liquid leaving the reservoir serves to increase the refrigerant charge in the condenser. An increase of refrigerant charge of the condenser results in more of the condenser being filled with liquid refrigerant. This reduces the heat transfer surface available for condensation of refrigerant and causes the condensing pressure to rise. denser side of expansion valve 12 is not permitted to drop below a predetermined value set by the saturation pressure at the temperature of reservoir 16, refrigerant continues to flow through valve 12 in a normal manner. Consequently, by maintaining at least a minimum pressure on the condenser side of expansion valve 12, a suitableflow of refrigerant into evaporator coil 13 is assured. It can be seen, therefore, that evaporator coil 13 will not be starved for refrigerant and consequently, both the capacity and temperature of the evaporator coil will be maintained within design conditions. However, flooding of the evaporator coil is prevented by operation of the thermostatic expansion valve since bulb 15 will sense when an excess of refrigerant has been supplied to the coil and will tend to move diaphragm 14 of expansion valve 12 to a position such that the flow of refrigerant through the valve is reduced.

If the temperature of the air surrounding condenser 11 begins to rise, the normal condensing pressure will tend to rise also thereby forcing more refrigerant back into reservoir 16 and maintaining a constant minimum pressure on the condenser side of expansion valve 12 at ambient temperatures which would produce condensing pressures below that for which expansion valve 12 was designed to operate. The level of refrigerant in reservoir 16 will rise and fall to maintain at least the minimum predetermined pressure on the high pressure side of the system.

An additional advantage to be gained from a refrigeration system of the type described is that a convenient means of limiting high head pressures is made available. If the condenser employed in the system is of sufiicient .size to provide a substantial amount of subcooling under normal operating conditions, and if the refrigerant reservoir is normally only partially filled, then upon a rise in condenser ambient temperatures, the accompanying higher head pressure will begin to force refrigerant into the reservoir thereby backing up less refrigerant at the liquid end of the condenser. Consequently, the condenser has more effective condensing surface and the head pressure will remain relatively constant.

It will be observed that the use of the reservoir in the manner described is limited to systems employing expansion valves. If, for example, a capillary tube were to be substituted for the thermostatic expansion valve of this invention, the resulting high pressure and large sub cooling of refrigerant leaving the condenser would cause the refrigerant to pass freely through the capillary tube thereby endangering the compressor by flooding the evaporator under the extremes of low temperatures for which this system is designed to operate effectively.

It will be seen that by the practice of this invention, the design of a refrigeration circuit which is required to Since the pressure on the con-.

operate satisfactorily under abnormally low ambient con ditions is greatly simplified. In fact, a refrigerant reservoir may be installed in pre-existing refrigeration circuits which have been found to require better refrigerant control than available by use of a simple thermostatic expansion valve of conventional design. In either new or modified existing refrigeration circuits the inclusion of a constant temperature reservoir of the type described maintains at all times at least a minimum refrigerant pressure on the condenser side of the expansion valve. Consequently, the tendency of an evaporator coil to ice in such a refrigeration system is greatly reduced. At the same time, the installation of the refrigerant reservoir does not endanger the operation of the compressor because the effectiveness of a thermostatic expansion valve in protecting the compressor from over feeding of the evaporator is fully maintained.

It will be understood that this invention is not limited to the described embodiment, but may be otherwise practiced within the scope of the following claims.

I claim:

1. A refrigeration system adapted for use under a wide variation in ambient condenser temperatures, said system comprising a compressor, a condenser, a variable orifice expansion device, and an evaporator, each connected to form a refrigeration circuit, said condenser and said expansion device being connected by a liquid line; means controlling the orifice size of said variable orifice expansion device in response to superheat sensed at the evaporator outlet; a refrigerant reservoir; passage means connecting said refrigerant reservoir to the outlet of said condenser, said passage means passing refrigerant liquid in either direction between said condenser and said refrigerant reservoir during operation of said compressor to permit liquid refrigerant from said refrigerant reservoir to back up in said condenser and maintain a minimum condensing pressure therein under conditions of relatively low condenser ambient temperature; and means to maintain a substantially constant predetermined temperature in said refrigerant reservoir, said predetermined temperature corresponding with a minimum condensing pressure which it is desired to maintain in said refrigeration circuit so that said variable orifice expansion device is enabled to function normally under conditions of low condenser ambient temperatures.

2. A refrigeration system as defined in claim 1 wherein said refrigerant reservoir is located in heat exchange relation with an area of substantially constant temperature thereby maintaining the refrigerant therein at a substantially similar temperature to that of said area.

3. A refrigeration system as defined in claim 1 wherein said refrigerant reservoir is located in an insulated container and has heating means associated therewith, said heating means including a thermostat responsive to the temperature of refrigerant in said tank and serving to maintain the refrigerant therein at a substantially constant temperature.

4. A refrigeration system adapted for use under conditions of widely varying condenser ambient temperatures, said system comprising a compressor, a condenser having an exterior surface which is subject to varying ambient temperatures, a thermostatic expansion valve and an evaporator each connected to form a refrigeration circuit, said condenser and said expansion valve being connected by a liquid line; a refrigerant reservoir substantially insulated from said ambient temperatures; passage means connecting said refrigerant reservoir to the outlet of said condenser, said passage means passing refrigerant liquid in either direction between said condenser and said refrigerant reservoir during operation of said compressor to permit liquid refrigerant from said refrigerant reservoir to back up in said condenser and maintain a minimum condensing pressure therein under conditions of relatively low condenser ambient temperatures; and heating means associated with said refrigerant reservoir for maintaining the reservoir at a relatively constant temperature regardless of the ambient temperature about said condenser.

References Cited in the file of this patent UNITED STATES PATENTS 2,359,595 Urban Oct. 3, 1944 2,518,212 Wilson Aug. 8, 1950 2,715,317 Rhodes Aug. 16, 1955 2,836,965 Kleist June 3, 1958 2,882,695 Zwickl Apr. 21, 1959 2,943,457 Wile et a1. July 5, 196i) 

